1. Field of the Invention
This invention relates to the refining of metals containing residual metals as impurities.
2. Description of the Prior Art
Many metals include small amounts of other metals, as residual metals, following smelting and preliminary refining operations. Scrap metals from various sources may also comprise residual metals.
The presence of residual metals often deleteriously affects the properties or workability of the contaminated metal, and removal of such residual metals is desirable.
It has previously been proposed to remove residual metals from molten steel by vacuum treatment. See, for example, Salomon-de Frieberg and Davenport, `Vacuum Removal of Copper from Melted Steel Scrap`, The Metallurgical Society of CIM, Annual Volume, 1977; Harris and Davenport, `Pilot Plant Scale Vacuum Distillation of Liquid Steel to Remove Copper`, Canadian Metallurgical Quarterly, Volume 18, 1979; Harris and Davenport, `Vacuum Distillation of Liquid Metals: Part 1, Theory and Experimental Study`, Metallurgical Transactions B, Volume 13B, December 1982, pages 581-591; and U.S. Pat. No. 4,227,922.
The work outlined in these prior references was particularly concerned with removal of residual metals from molten steel as distinct from other metals.
In U.S. patent application Ser. No. 367,262, the applicants disclosed the refining of steel containing residual metals and impurities and apparatus for this purpose. An essential feature of the described refining process is the subjection of the liquid steel containing metallic impurities in a bath, to a vacuum effective to cause emission from the liquid steel surface of the metallic impurities as a flow of rising gases. According to the invention, the surface of the molten steel is maintained substantially free of surface contamination and return of the residual metals into the molten steel is continuously prevented. Vacuum is maintained, throughout the treatment, at a level at which the emission is in the form of a bulk flow of rising gases which, in combination with the continual freeing of the surface of contamination and the continual prevention of the return of the impurities, reduces the time of the removal of impurities substantially to a minimum.
The applicants have also disclosed the use of a lifting gas, to lift liquid steel from a lower level in the bath to above the surface of the bath, and discharging the lifted steel to the surface of the bath to maintain it free of contamination. This latter expedient also has the advantage of exposing a large surface area of the lifted steel to the vacuum.